Praise be to Allah.
If the families are living all together, then the women must observe hijaab in front of those who are not their mahrams. It is not permissible for the wife of one brother to uncover her face in front of his brother, because his brother is like any other man in the street with regard to looking and being non-mahram. It is also not permissible for his brother to be alone with her if he (the husband) is outside the home. This is a problem which is affecting many people, such as when brothers are living in the same home and one of them is married. It is not permissible for the one who is married to leave his wife with his brother if he goes out to work or to study, because the Prophet (peace and blessings of Allaah be upon him) said: “No man should be alone with a [non-mahram] woman.” And he said: “Beware of entering upon women.” They said, “O Messenger of Allaah, what do you think about the brother-in-law [meaning the husband’s relatives]?” He said, “The brother-in-law is death.”
There are always questions about the crime of adultery (zina) in such cases, where a man goes out and leaves his wife and his brother in the house, then the Shaytaan tempts them and he commits adultery with her – Allaah forbid – with his brother’s wife. This is worse than committing adultery with his neighbour’s wife, indeed it is more terrible than that. I want to say something that so that I will have discharged my responsibility before Allaah. It is not permissible for a man to leave his wife in the same house as his brother, no matter what the circumstances, even if his brother is one of the most trustworthy, sincere and righteous of people, for the Shaytaan flows through the son of Adam like blood, and sexual desire knows no bounds, especially among young people.
But what should we do if two brothers are living in the same house and one of them is married? Does that mean that when he goes out he must take his wife out to work with him?
The answer is no, but the house may be divided in two, one half for the brother on his own, with a door that can be locked, for which the key should be kept by the husband when he goes out to work. So the wife will be in a separate part of the house, and the brother will be in another separate part of the house. But the brother may argue with his brother and say, “Why are you doing that? Don’t you trust me?”
The answer to that is to tell him, “I am doing this in your own best interests, because the Shaytaan flows through the son of Adam like blood. Maybe he will tempt you and desire may overwhelm reason, in which case you will do something haraam. I am doing this to protect you. It is in your best interests as well as mine.” If he gets angry because of this then let him get angry and do not worry about him.
I am telling you these things so as to discharge my responsibility before Allaah, and your reckoning will be with Allaah.
With regard to uncovering the face, it is haraam; it is not permissible for a woman to uncover in front of her husband’s brother because he is a stranger (non-mahram) to her, so he is just like any other man in the street.